1. Field of the Invention
This invention relates to aluminum rigid container sheet and an improved method for cleaning the same. The invention further relates to an improved cleaner composition for beverage can sheet, particularly can end sheet made from 5000 Series aluminum alloys (Aluminum Association designation). In addition, this invention addresses the specific need to lower the magnesium concentration on the aluminum oxide surface and throughout the oxide-aluminum metal interface of such rolled sheet product, preferably by modifying said surface and interface and depositing a corrosion inhibitor thereon.
2. Technology Review
Brasko et al. U.S. Pat. No. 3,962,060, the disclosure of which is fully incorporated herein by reference, discloses a continuous system for electrodepositing a coating material on metal sheet. That system includes: uncoiling and cleaning said sheet; passing it in a substantially straight line through respective coating, baking and cooling stations; supporting the sheet in a substantially straight position while directing liquid coating materials against opposite surfaces of said sheet; and electrically contacting this sheet with at least one charged electrode to promote the deposition of coating material on said sheet product. The present invention improves the cleaning steps of the foregoing electrocoat/electrodeposition system though it is to be understood that it may also be used in conjunction with other aluminum treatment processes, including the extrusion coating process set forth in U.S. Pat. No. 5,407,702, the disclosure of which is also fully incorporated by reference herein.
Numerous means are known for "cleaning" the surfaces of aluminum materials, especially those used for making food and/or beverage container products. Representative aluminum cleaning processes and/or compositions have been described in U.S. Pat. Nos. 3,687,858, 4,126,483, 4,528,039, 4,540,444 and 4,762,638. Alkaline-based cleaners for steel sheet products are taught in U.S. Pat. Nos. 4,010,086 and 4,382,825.
Gregory et al U.S. Pat. No. 4,778,533 focuses on a method for chemically treating aluminum-magnesium alloy sheet product to substantially inhibit magnesium oxide-containing films from forming thereon. The method comprises chemically treating such sheet, without prior degreasing or etching, with a polycarboxylic acid-free, alkaline-based cleaner to which is added more than about 0.2 wt. % of a compound containing hydroxyethylidene-1,1-diphosphonic acid, or HEDP.
For purposes of preventing de-smutting and blackening of a 3000 Series aluminum can body stock which contains very little magnesium, Aoki et al. U.S. Pat. No. 5,382,295 uses a four-part cleaner consisting of: from 0.5 to 10.0 g/L of an alkali builder component; from 0.5 to 10.0 g/L of an aminoalkylphosphonic acid, hydroxyalkylphosphonic acid, or water soluble salt thereof; from 0.1 to 3.0 g/L of an aluminum ion sequestering agent; and from 0.5 to 5.0 g/L of a surfactant component, as part of a 10-12 pH, aluminum cleaner used to contact said stock for 20-60 seconds, or more typically for 30-50 seconds, before a lacquer is applied directly thereto.